North Korea has warned of a nuclear war on the Korean peninsula while vowing to step up its atomic weapons programme in defiance of new UN sanctions.

Today's Rodong Sinmun, a state-run North Korean newspaper, claimed the US has 1,000 nuclear weapons in South Korea. Another state-run publication claimed that America had been deploying nuclear weapons in Japan as well.

North Korea "is completely within the range of US nuclear attack and the Korean peninsula is becoming an area where the chances of a nuclear war are the highest in the world", the Tongil Sinbo said.

A spokesman at the US military command in Seoul dismissed the claims as "baseless", saying Washington had no nuclear bombs in South Korea. US tactical nuclear weapons were removed from the country in 1991 following the cold war.

Yesterday, Pyongyang threatened war on any country that dared to stop its ships under the new sanctions approved by the UN security council on Friday.

Pyongyang's sabre-rattling presents a growing diplomatic headache for Barack Obama as he prepares for talks on Tuesday with his South Korean counterpart on the North's missile and nuclear programmes.

President Lee Myung-bak told security ministers at an unscheduled meeting today to "resolutely and squarely" cope with the North's latest threat, his office said. He leaves for the US tomorrow morning.

South Korea's unification ministry today demanded that the North stop stoking tension, abandon its nuclear weapons and returned to dialogue with the South.

It is unclear whether North Korea's statements are simply rhetoric. But they are a setback for international attempts to rein in the country's nuclear ambitions following its second nuclear test on 25 May.

In yesterday's statement, Pyongyang said it has been enriching uranium to provide fuel for its light-water reactor. It was the first public acknowledgment that the North is running such a programme in addition to its known plutonium one.

Today, Seoul's Yonhap news agency reported that South Korea and the US have mobilised spy satellites, reconnaissance aircraft and human intelligence networks to obtain evidence of the programme.

North Korea says its nuclear programme is a deterrent against the US, which it routinely accuses of plotting to topple its regime. Washington, which has 28,500 US troops stationed in South Korea, has repeatedly said it has no such intention.

The latest UN sanctions are aimed at depriving Pyongyang of the financing necessary for its nuclear programme. The UN also authorised searches of North Korean ships suspected of transporting illicit ballistic missile and nuclear materials.The new sanctions

The UN penalties provided the necessary tools to help check North Korea's pursuit of nuclear weapons, said the US secretary of state, Hillary Clinton yesterday.

They show that "North Korea's pursuit of nuclear weapons and the capacity to deliver those weapons through missiles is not going to be accepted by the neighbours as well as the greater international community", she said.